Abstract Nearly every form of cardiac disease involves changes to the myocardial stiffness. Current clinically-viable measurement techniques to assess myocardial stiffness are ei- ther invasive and/or involve image-derived metrics. We propose the use of a new imaging method, called Acoustic Radiation Force Impulse (ARFI) imaging, in measuring the me- chanical properties of myocardial tissue. The technique employs ultrasonic radiation force to displace tissue. Conventional ultrasound is then used to observe the response of the tissue to the force. We present preliminary data showing the success of this technique in imaging the changes in stiffness that occur under normal myocardial function. We pro- pose to further develop the ARFI imaging technique and then investigate the use of ARFI imaging in detecting abnormal heart function using animal models. In addition, we pro- pose human clinical studies with pediatric cardiac transplant patients. These studies will evaluate the potential of ARFI imaging to provide early detection of transplant rejection, in grading the severity of heart failure, and in distinguishing between systolic and diastolic heart failure.